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01-02-2011, 10:42 AM   #316
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Source Color Temp Is Not the Issue With My K5

QuoteOriginally posted by falconeye Quote
Ray, thanks for your extensive testing.

The post I found most useful was this: Re: Low light level and focussing for K-5: Pentax SLR Talk Forum: Digital Photography Review

You demonstrate that there is a threshold where the camera misses focus. For both tungsten and daylight. However, I didn't find a test where you studied how this threshold depends on color temperature.

Maybe I missed it as the discussion in the other thread is rather confusing with various theories and actual results mixed.

I've seen Gordon's theory now (a failure to measure color temperature) and it isn't my preferred one. It's not impossible but not very plausible. Color temperature can normally be measured more accurately than focus because you normally have a larger light sensitive surface per measured number. OTOH, I don't know how the +-sensor is implemented. So, Gordon's theory is a possibility.

Overall though, I think we're speculating far too much (in that other thread).


I still think we at least have to conduct the following experiments before jumping to speculations:

1. Amount of defocus as a function of illumination (your test).
2. Amount of defocus as a function of illumination for another color temperature (your test).
3. Derivation of color temperature dependency (my test shows that this dependency is large too). One could do a test in red, green and blue light...
4. A test with a second body to study sample variation.


Personally though, I rather wait for Pentax doing their homework and will observe the results with interest
Falk,

I shot several tests:

Tungsten.

Full daylight.

Stepped light reduction in Tungsten.

I used my WhiBal cards to shoot several images to use in assessing color temp:

Some with no ND filters, some with ND filters in the same lighting. I then opened the images in ACR and used the color dropper tool to measure the color temp of the white card.

In the tungsten shots I measured 2900K with no ND and just a bit lower with the NDs (the filters are not 100% neutral). The focus shifted to FF in 2900K light as the light through the lens and into the AF system was reduced by the filters.

In the daylight shots, I measured 5000k with no ND and just a bit lower with the ND using the same WhiBal method. The focus shifted to FF in 5000K light as the light through the lens was reduced by the filter.

I then set up my K5 with a test target in tungsten light (it is raining here the last two days) and shot images with stepped light reduction of 2,3,4,5,6,7,8 stops in the same lighting. The focus clearly shifts to FF as the light is reduced with no change in color temperature.

Lastly, look at my videos of the K5 shifting as I put the ND filters over the lens. Keep in mind that the ND filters are simply reducing the light entering the lens in the same color temperature of light.

So, I have several data points that tell me failry conclusively that my K5 shifts as the light is reduced regardless of color temperature.

As far as I am concerned, for my K5, there really is no argument that will convince me that color temp is the problem. I can make my K5 shift to FF in any color light by simply reducing the amount of light entering the system. While other bodies might not act the same way, I would LOVE to see a decent test of such a body.

I could be convinced that the SAFOX+ color temp sensor/algorithm is the problem as I can imagine how this sub-system could give us results that look like a color temp issue when it is really a low light issue with the + sub-system.

3rd K5 body on the way...

Fingers crossed for no sensor stains and proper focusing. If Pentax swings and misses a 3rd time, I will need to assess my options and decide what to do.

Ray

01-02-2011, 12:24 PM   #317
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QuoteOriginally posted by Ray Pulley Quote
Keep in mind that the ND filters are simply reducing the light entering the lens in the same color temperature of light.
I would like to have such a ND filter. Inspite of the name ''Neutral Density'' i havent seen such kind of filters. All ND filters shifts more or less or change color temperature.... there is no really any other way to reduce the light amount than diaphragm or aperture without changing the spectrum of light
01-02-2011, 12:52 PM   #318
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PD AF sensors that are put into the image sensors will solve all problems currently appearing with DSLR's in respect of BF and FF. Just keep your money in the pocket for a few months more and you will find such more advanced systems of AF on the market. Before March Nikon and Canon will introduce their mirrorless systems that are rumoured to be professional level. Wait and see but one thing is for sure 2011 will bring us much more confident AF shooters in the K5 class ! Just sit on your money a little while more guys !
In case you want to shoot with very secure AF take a cheaper "in between " solution like a canon S95 for example . ALso the upcoming OLYMPYS XZ-1 will do well in low light and most likely with good AF. Bigger sensor alternatives to the K5 will have arrived by the time the joy of the smaller cams is over for you. Flipping mirrors are from an other era sooner than we expect I am afraid and AF measuring will be done differently direct on the main sensor sooner than later. I would suggest that hardware should not be bought with a long term view as developments of alternativ (better) technology has just started to pick up pace.
01-02-2011, 01:09 PM   #319
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No Disagreement

QuoteOriginally posted by filorp Quote
I would like to have such a ND filter. Inspite of the name ''Neutral Density'' i havent seen such kind of filters. All ND filters shifts more or less or change color temperature.... there is no really any other way to reduce the light amount than diaphragm or aperture without changing the spectrum of light
I agree, although some ND filters are better than others in this regard.

What you are missing is that I specifically measured the color temp with and without the ND filters mounted by shooting an image with the filters mounted, and while this method isn't perfect, I can at least see the difference in what the camera captured which is all that matters. Note also that in testing the AF, I performed a focus only cycle with the ND filters on, but removed them for the actual exposure, so the camera exposed the image with no filters on the camera. You can set your K5 to NOT lock exposure with focus lock, whcih is what I did. This way, the only difference between one image to the next was that I locked focus with and without the filters. All shots were raw so I could measure the actual color temp as captured by the camera.

This is as close as I can get to controlling the variables with the tools I have, and it confirms what I was also seeing in real life images. YMMV.

The amount of FF shift I am seeing is very large as the light is reduced, and this cannot be explained by the minor change in color temperature induced by the filters. If, in fact a 200 - 300 unit shift in color temp (starting at ANY color) causes this sort of FF focus shift, then the camera will be all but unusable in any lighting conditions as color temp changes constantly in any shooting situation.

Whatever the cause, my K5 is pretty useless at EV 4-5 or below regardless of temperature. Hopefully body #3 will be one of the "perfect" ones with no stains or AF issues, but I suspect that it will FF more or less like this one does.

Ray


Last edited by Ray Pulley; 01-02-2011 at 01:11 PM. Reason: Typo
01-02-2011, 03:44 PM   #320
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QuoteOriginally posted by Ray Pulley Quote

So, I have several data points that tell me failry conclusively that my K5 shifts as the light is reduced regardless of color temperature.

As far as I am concerned, for my K5, there really is no argument that will convince me that color temp is the problem. I can make my K5 shift to FF in any color light by simply reducing the amount of light entering the system. While other bodies might not act the same way, I would LOVE to see a decent test of such a body.
Ray, thanks for providing a summary of your work. It confirms what I thought.

Nevertheless, and please apply some tight arguments yourself, you only showed that the FF shift emerges in low light for both, daylight and tungsten. You did NOT show that the issue is independent on color temperature.

Esp. with the ND filter and their strong filtering, you can only say the issue emerges. But not at what EV values exactly.

So, if I combine your test and my test, I would say that the issue emerges at any color temperature at a low enough light level threshold. But this threshold is lowest for daylight, highest for halogen, and somewhere in between for tungsten. Which is strange enough and should give Pentax a clue.

Personally, I still think the color temperature correction factor runs away in low light, either by imprecise measurement (Gordon's idea) or some software bug where, e,g., a register flows over. Or the AF module measurement time isn't raised enough in low light with a fast lens. It would be interesting to study the sample variation here though.
01-02-2011, 05:46 PM   #321
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I Guess We Will Have to Agree to Disagree

QuoteOriginally posted by falconeye Quote
Ray, thanks for providing a summary of your work. It confirms what I thought.

Nevertheless, and please apply some tight arguments yourself, you only showed that the FF shift emerges in low light for both, daylight and tungsten. You did NOT show that the issue is independent on color temperature.
Huh? I shot images of my White Balance cards with and without the filters and measured the color shift with the filters on as captured by the camera. There was little color shift with the filters on, leaving just the reduced light entering the lens as the element that changed in each test.

The same test was repeated for a lower color temp with the same FF shift result.

It is true that I could not get anywhere near as much light on the subject with Tungsten as I could in full daylight (I left my portable fusion light source at work ), so the camera was already somewhere along the curve to FF in the tungsten test.

Reducing the light and NOT the color temperature caused FF in all of my tests. The color temp of the source clearly did not matter.

I also shot several tests in Tungsten with an 80A filter on the lens and the FF shift is exactly the same as the light is reduced.

At the end of the day, it doesn't matter what the cause is. The camera simply doesn't focus properly as the light is reduced regardless of the cause.

QuoteQuote:
Esp. with the ND filter and their strong filtering, you can only say the issue emerges. But not at what EV values exactly.
I stepped the test one ND filter at a time and the FF starts somewhere around EV 3-4. I have 2 and 3 stop ND filters, so I could reduce the light one stop at a time starting at 2 stops. The progression was pretty clear, but there is a steep drop-off at the low end to massive FF. The drop off might well be below the range of the AF system, but it did not fail, it locked focus consistently in significant FF. The AF assist light also never came on, interestingly enough.

QuoteQuote:
So, if I combine your test and my test, I would say that the issue emerges at any color temperature at a low enough light level threshold. But this threshold is lowest for daylight, highest for halogen, and somewhere in between for tungsten. Which is strange enough and should give Pentax a clue.

Personally, I still think the color temperature correction factor runs away in low light, either by imprecise measurement (Gordon's idea) or some software bug where, e,g., a register flows over. Or the AF module measurement time isn't raised enough in low light with a fast lens. It would be interesting to study the sample variation here though.
The cause could be in a number of areas in the design of the camera, I agree. However, if there is a software bug or a consistent offset error that is in the design and not the result of some other component alignment, adjustment or sensitivity tolerance, then every K5 has to have the same problem.

I will have another sample to test on Tuesday.

Ray
01-02-2011, 06:11 PM - 1 Like   #322
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Failure to Communicate?

QuoteOriginally posted by falconeye Quote
Ray, thanks for providing a summary of your work. It confirms what I thought.

Nevertheless, and please apply some tight arguments yourself, you only showed that the FF shift emerges in low light for both, daylight and tungsten. You did NOT show that the issue is independent on color temperature.
Ok, perhaps we are just not communicating clearly?

If you are saying that I did not show that a color temp change also causes a focus shift, you are correct. I was trying to control for color and vary the light level only. The tests conclusively show that for my K5, reduced light creates a FF shift. It could be that color also causes a color shift, but so far any such shift is not great enough to offset the low light shift. It could be adding to the low light shift, however.

I might be able to setup a test using the 80A, FLW, FLD, red, yellow and a few other filters I have available that change the color of the light entering the lens. I would have to control for the light level however, as most filters reduce the light by some amount and not equally. That means that I would have to try and vary the light level so that the amount of light entering the lens is the same after the filter is applied.

Frankly, if the next body shows serious FF in low light in any shooting tests I do, I will likely not bother to test any color shift as it will be a waste of time. The K5 will be going back at that point and I will be thinking about a change to another brand versus waiting to see if the K5 gets sorted out.

Ray
01-02-2011, 06:18 PM   #323
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All I can say..Is thank god we have some very clever Blokes using the K5..and and taking the time to do all these tests....you lot need a medal.

Ill sit here watching events unfold...

would a simple fix of having the AF assist light come on earlier do the Job ?..not much help for AFC mode though I guess?

just sitting at my PC doing some quick and dirty testing...in 50% of my shots the AF lamp comes on..Perfect Focus..the other 50%..no lamp..no focus...perhaps we need a pocket torch to shine on the subject ourselves ..LMAO

01-02-2011, 06:32 PM   #324
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QuoteOriginally posted by bjan Quote
PD AF sensors that are put into the image sensors will solve all problems currently appearing with DSLR's in respect of BF and FF. Just keep your money in the pocket for a few months more and you will find such more advanced systems of AF on the market. Before March Nikon and Canon will introduce their mirrorless systems that are rumoured to be professional level. Wait and see but one thing is for sure 2011 will bring us much more confident AF shooters in the K5 class ! Just sit on your money a little while more guys !
In case you want to shoot with very secure AF take a cheaper "in between " solution like a canon S95 for example . ALso the upcoming OLYMPYS XZ-1 will do well in low light and most likely with good AF. Bigger sensor alternatives to the K5 will have arrived by the time the joy of the smaller cams is over for you. Flipping mirrors are from an other era sooner than we expect I am afraid and AF measuring will be done differently direct on the main sensor sooner than later. I would suggest that hardware should not be bought with a long term view as developments of alternativ (better) technology has just started to pick up pace.
This post shows the weakness of single metric application to a product choice.
What Nikon, Canon or Sony put on the market is of no interest to someone who is fairly heavily invested in a lens family.
A person who buys a body and a kit lens can change brands on a whim, a person who buys based on a brand investment just sees a tremendous lack of thinking in posts such as the one I quoted.
01-02-2011, 07:05 PM   #325
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FWIW, I've seen folks on other brands post about how UV filters can help cause FF/BF. Not sure the ND test is valid in other words...
01-02-2011, 07:20 PM   #326
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It is Valid Enough

QuoteOriginally posted by kenyee Quote
FWIW, I've seen folks on other brands post about how UV filters can help cause FF/BF. Not sure the ND test is valid in other words...
Not sure what anecdotal remarks by some third party about some unspecified UV filter have to do with this, but thanks anyway.

Note that many, myself included, are seeing FF shifts with NO filters, and this same shift can be seen by reducing the light level with filters, or by turing down the light (you know like when the sun starts to go down and stuff like that).

No filters/low light - FF

Filters - FF

Strange coincidence don't you think?

Ray
01-02-2011, 07:42 PM - 3 Likes   #327
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Here Ya Go

QuoteOriginally posted by falconeye Quote
Ray, thanks for providing a summary of your work. It confirms what I thought.

Nevertheless, and please apply some tight arguments yourself, you only showed that the FF shift emerges in low light for both, daylight and tungsten. You did NOT show that the issue is independent on color temperature.

Esp. with the ND filter and their strong filtering, you can only say the issue emerges. But not at what EV values exactly.

So, if I combine your test and my test, I would say that the issue emerges at any color temperature at a low enough light level threshold. But this threshold is lowest for daylight, highest for halogen, and somewhere in between for tungsten. Which is strange enough and should give Pentax a clue.

Personally, I still think the color temperature correction factor runs away in low light, either by imprecise measurement (Gordon's idea) or some software bug where, e,g., a register flows over. Or the AF module measurement time isn't raised enough in low light with a fast lens. It would be interesting to study the sample variation here though.
All shot with the K5 in Raw, tungsten light source (it is dark here now), FA 50 f1.4, exposure equalized as much as possible with the filters on by moving the lamp to get as close as possible to the same shutter speed at ISO 100 f2.0:



Image on the left was shot through an 80A filter (blue, ACR color dropper on whiBal card says 5850 color temp)

Image in the middle was shot through a Y2 filter (Yellow, ACR color dropper on whibal card says 2850 color temp)

Image on the right was shot through R2 filter (Red, ACR color dropper on whibal card says 2000 color temp, but I think that this is the lower limit of ACR, so the color temp is probably lower than that).

I did not try and remove the resulting color casts so as to make which one is which more clear.

Note that all are a bit back-focused, which is probably where this lens is calibrated in bright light), and that if anything the focus shifts a bit to the back from blue to red. This could well be just normal AF shot to shot tolerance, however.

Note also the somewhat fuzzier appearance as the images move from blue to red. This is EXACTLY what I would expect CA from the lens to produce as we move towards near IR. The red is not as well corrected and focused by the lens, which produces lower contrast and a softer appearance, but did not result in any significant focus shift, and certainly not to FF.

Ray
01-03-2011, 12:36 AM   #328
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Well done.

(rep added)
01-03-2011, 09:57 AM   #329
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QuoteOriginally posted by Ray Pulley Quote
Note that many, myself included, are seeing FF shifts with NO filters, and this same shift can be seen by reducing the light level with filters, or by turing down the light (you know like when the sun starts to go down and stuff like that).
I didn't say there wasn't a problem..just that filters *may* influence results...
01-03-2011, 10:38 AM   #330
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I have been reading all of this, and like a few others here that are not of a technical inclination regarding electronics, I don't understand much of the discussion in technical terms. I just take photos and hope I get a few right, and sometimes I do get lucky......mostly not.

I have been wanting the K5 since it was announced, but like others, I am not wanting to play the back and forth return game, I just want to use the camera I get without a problem other than me, which is usually sufficient for enough frustration.

So, can someone answer my concerns to the best of their ability? Does this affect MF at low light levels? What I am reading is that we finally have a camera that is superb at low light shooting with excellent High ISO results, what I am looking for, but you can't focus with it unless you make continuous back and forth adjustments of lenses, which may or may not solve the problem? Is this a wrong assumption, right assumption, or did I just get it all wrong? It appears that this problem is universal, yet a few do not report a problem in this area...I do not understand how this can be? Can that be explained?

The sensor stain is not an issue, easily found and repaired/replaced, but a camera made for low light shooting among other attributes, that won't focus in low light....that is a problem I don't want.
Thanks in advance for any replies you can give.
Rupert
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